Summary

Could dark matter consist of primordial black holes, as numerous as the stars? It's an old, improbable idea, but it made a Lazarus-like comeback a year ago, when the discovery of gravitational waves suggested that the cosmos abounds with unexpectedly heavy black holes. Last February physicists with the Laser Interferometer Gravitational-Wave Observatory (LIGO) announced that they had detected ripples in space from the violent merger of two black holes 29 and 36 times as massive as our sun—more than twice as massive as physicists thought so-called stellar mass black holes could be. If scads of those black holes are out there, then they might account for the 85% of the universe's matter that is missing, one team of physicists argues. However, the idea is now coming under pressure from other directions, including studies of the cosmic microwave background and of tiny dwarf galaxies on the periphery of the Milky Way. And a definitive census of black holes might come in a few years, not from LIGO, but from studies of mysterious fast radio bursts and pulsars.